Preheat the oven to 160°C fan / 180°C conventional.
Lightly grease a 2lb loaf tin (I used this one by Circulon) and line with greaseproof paper (see notes in the post above for how to do this). Set aside for now. Cream the margarine, golden caster sugar and vanilla extract together in a large bowl until pale and fluffy.
Mix in the eggs one at a time, adding 2 tablespoons of the 300g flour after each egg to stop the mixture from curdling.
Add the remaining flour, lemon zest and milk. Fold the mixture together until smooth and fully combined.
Toss the raspberries in a teaspoon of flour (this will stop them from sinking to the bottom of the cake) and then fold them into the mixture.
Transfer to the loaf tin and bake for 60-70 minutes* until golden in colour and a toothpick inserted into the middle comes out clean.